Techniques for measuring wind velocity and direction using so-called Doppler radar are well known in the art. Atlas, U.S. Pat. No. 4,649,388, issued Mar. 10, 1987 and incorporated herein by reference, discloses one such Doppler radar for measuring low level wind shear. The problem of wind shear in aviation is well known, and application of such Doppler radars for measuring and detecting wind shear is well known in the art.
However, such radar systems may also be adapted for other uses as well. For example, measurement of wind velocities and directions at various altitudes may be useful in researching weather patterns, measuring air pollution, and the like. Jordan, U.S. Pat. No. 5,592,171 issued Jan. 7, 1997, and incorporated herein by reference, discloses such a wind profiling radar. The Jordan system provides a method for reducing the amplitude of a radar return signal from the ground and other sources of "clutter"; reflections from the ocean surface, birds, airplanes, and precipitation.
Such systems are also described in Clifford et al., Ground-Based Remote Profiling in Atmospheric Studies: An Overview, PROCEEDINGS OF THE IEEE, Vol. 82, No. 3, March 1994, also incorporated herein by reference.
In some instances, it may be desirable to mount such Doppler radar systems on moving platforms. For example, to measure weather patterns or detect wind shear over an ocean area, it may be desirable to mount a Doppler radar antenna on a ocean buoy or the like. Such a buoy mounting may be desirable to measure movement of pollution over the water, or for detecting wind movement near an ocean- or lake-side airport.
In addition, there may be other applications where a Doppler antenna maybe subject to movement, either intentionally or unintentionally. For example, an antenna may be mounted on a ship or airplane for measuring wind and weather patterns. In the prior art, it is known to provide gimbaled antenna systems for maintaining an antenna in a predetermined position regardless of vehicle movement. However, such antenna systems are cumbersome, expensive and heavy, which may be a disadvantage particularly in small craft (boats) and aircraft.
Moreover, such gimabaling systems may only provide motion compensation for a limited range of movements. In heave seas or other rough conditions or movements (e.g., sudden turn in small boat or airplane) such a system may not be able to compensate, and the antenna may run against the "stops" of the gimbal.
In other applications, it may be intended to maintain an antenna stationary, however, other factors may cause some slight movement in the antenna. For example, a tower-mounted antenna may tend to sway in the wind, introducing artifacts in a received signal.
When operating a wind profiler on a buoy (or any moving platform) motion of the buoy or platform produces spectral broadening. A wind profiler requires about 30 seconds of averaging time to reduce noise variance enough to detect a clear air signal. A buoy, for example, can sway back and forth through several cycles during a 30 second averaging time.
Buoy motion during the averaging time will broaden the clear air peak in the Doppler spectrum causing errors in the measured power (zeroth moment) radial velocity (first moment) and spectral width (second moment). Buoy motion is slow enough that it can be assumed to be negligible for a short time. Unfortunately, the time the buoy position can be assumed to be fixed does not in general allow enough averaging to detect the clear air signal in the Doppler spectrum.